Chemical milling



7;. QM/7M M, it/@+Mww June 14, 1960 H. B. sNYDER ErAL 2,940,838

Cl-IEMICAL MILLING Filed Aug. 19, 1957 FIG.6

' design is silverware. to form a roughened surface. In order tospecifically in- United States Patent O CHEMICAL MILLING Herman BenSnyder and Ludwig M. Rosenberg, Seattle,

Wash., assignors to Boeing Airplane Company, a corporation of DelawareFiled Aug. 19, 1957, Ser. No. 678,882

8 Claims. (Cl. 4142) This discovery relates to the chemical milling ofmetal in an acid medium and, more particularly, to the chemical millingof age hardened, precipitation hardened and other heat treatablestainless steels in all their solid phases in an acid medium.

Prior to specifically discussing chemical milling we wish to point outthe manner in which it distinguishes from pickling, brightening,decorative design and surface increase. Chemical milling may beconsidered to be controlled corrosion or controlled metal removal toform Sculptured metal configurations. In chemical milling a relativelylarge percentage of the metal may be rapidly removed so as to leave aminor amount of the original metal in a new configuration. As contrastedwith this is pickling or scale removal whereby as much as possible ofthe oxide and other coating of the metal are removed but as small amountas possible of the metal is removed. In other words in pickling only thesurface coating of the metal is removed. In brightening or surfacepolishing a minimum amount of the metal is removed to form a reflectivesurface as the scale has been previously removed. Decorative designs areof a shallow depth with approximately five mils of surface removed bychemical action. In decorative design part of the surface is masked andthe design is inscribed through the masked coating. Next, a chemicalsolution is applied to the masked material and allowed to react with themetal to form the design. An example of decorative In surface increasethe object is crease the surface area the same is purposely pitted bychemical action. This is done to make the surface rough enough forbonding purposes such as a metal to metal bond, a base for paint and.the manufacture of condenser plates to name a few of the uses.

The mechanical milling of metal is well-known and extensively usedthroughout the metal working industry. Even though mechanical milling isextensively used it inherently has certain limitations. One of theselimitations is the expensive capital investment for milling machines.Such a machine, capable of making large, complicated configurations, maycost in the hundreds of thousands of dollars. In addition to the capitalinvestment it is necessary to have skilled workmen operating thesemachines in order to do a creditable job on the milling of metal. Also,because of cost and manhour requirements this type of milling is limitedto certain configurations, namely, those configurations which are of amore` simple or open design. However, by the skillful use of welding itis possible to build these limited configurations into an intricatedesign. Because of the nature of the weld and the inherent weaknesstherein it is not always advisable to build these limited basicconfigurations into an intricate design as the weld might rupture or theadded weight become excessive. Furthermore, with mechanical millingthere usually results a relatively rough surface. Because of excessivecost the tolerance, in a recessed area, is not a close one but must be arather large one such as in the hundreds of an inch. Also',- on some ofthe newer metals it is impractical, because of cost due to equipment andto the time of milling involved, to mechanically mill the surfaces asthe metal is extremely tough and resistent to mechanical cutting and/orrapid abrasion. For example, only casting and rough grinding can beemployed with certain metals.

In recent years, as a supplement to mechanical mill-V ing, there hasdeveloped chemical milling. One of the rst patents to be issued forchemical milling and the like is the patent to Wilson et al., UnitedStates Patent Number 2,684,291. Wilson et al. describe the emboss ing ofsteel with an acid medium. Basically, this patent presents the method oftaking flat stock, masking certain predetermined areas or portions ofthis stock with an acid resistant covering, and etching the masked stockwith the acidic solution. The acid solution eats away or corrodes awaythe unmasked areas to form the desired design. Naturally, it is possibleto remove the stock from the acid solution, remove the masking cover,add new masking cover, and insert the stock again in the solution. Bysuch means it is possible to form intricate designs in metal. With ourdiscovery for the use of a strong acid solution in chemical milling wefind it possible to realize a more rapid action than Wilson et al.realize with a weak acid solution; and to mill steel into predetermineddesigns and configurations while Wilson et al. are restricted to onlysurface change. Also, our strong acid milling solution makes it possibleto work with equal ease on martensitic steels be they age hardened,precipitation hardened, or heat treated stainless steels.

To further illustrate some of the advantages of chemical milling it ispossible to achieve the same output by chemical milling with a lowercapital investment than for an equal output with mechanical milling.Also, in chemical milling the application of masking material to thepart to be formed requires semi-skilled labor as contrasted with therequirement of skilled labor for mechanical milling. And, more intricateconfigurations can be formed more rapidly by chemical milling as themetal to be milled can be immersed in the reactant solution so that allparts of the metal can be acted upon simultaneously. In a specicillustration as applied to the manufacture of airplanes it is possibleto reduce the weight of certain members of the airplane, as an example,stainless stgel sheet stock may come in a thickness of 0.093 inch;however, from a design standpoint a thickness of 0.086 inch may beadequate. The original stock would not be reduced from the thickness of0.093 inch to the thickness of 0.086 inch by mechanical milling as thecost would be too great, especially for heat treated stock. However, itis possible by chemical milling to immerse the stock in a chemicalreactant solution and reduce it to the thickness desired with ease andeconomy. In large airplanes this reduction in thickness may mean aconsiderable saving in the weight of the air frame. More particularly,for every excess pound carried by the airplane there may be requiredfrom tive to ten pounds of additional gross air frame weight. Therefore,if in the larger airplanes a thousand pounds of unneeded material can besaved the air frame weight can be reduced by approximately 5,000 poundsto 10,000 pounds.

In another manner the configuration of a chemically milled metal can bemore economically achieved than by mechanical means. For example, in thecase of a wale pattern, the necessary chemical milling can be relativelyeasily performed as the masking materials can be placed over the metaland the metal not protected by this masking material can be chemicallyeaten away or chemically corroded. In a like manner an object can betapered by removing the same slowly from the chemical milling solution.Also, an object having a large number of irregularly placed recessedareas can be readily manufactured by chemical milling while thepreparation of a similar object by mechanical means would betime-consuming and ditiicult to prepare.

With this in mind it is an object of this discovery to provide a methodfor the chemical milling of metal, examples being intricateconfigurations, taper, and wallie pattern.

A further object is the provision of a method for the chemical millingof a metal and, particularly, for the chemical milling of age hardened,precipitation hardened and other heat treatable stainless steels in alltheir solid phases.

A further object is to provide a method for the chemical milling at arelatively uniform and controllable rate.

An additional object is to provide a solution for the chemical millingof metal and which solution can be controlled by standard methods ofchemical analysis.

Another important object is the provision of a process for the chemicalmilling of metal and which process provides satisfactory surfaces havingwell defined edges at better than standard machine tolerances.

A still further object is the provision of a process for chemicalmilling resulting in satisfactory structural and mechanical propertiesof the milled metal.

A still further and important object is to provide for the chemicalmilling and which chemical milling results in a satisfactory smoothsurface.

Another object is a provision of a process for chemical milling andwhich process produces a minimum of smut.

A further object is the provision of a process for chemical millingwhich is less expensive than mechanical milling and makes it possible toproduce more intricate configurations than mechanical milling.

In the drawings:

Figure 1 is a plan view of a curved structural member illustrating tworectangular-shaped recesses in the open surface thereof.

Fig. 2 is a longitudinal vertical cross-sectional view taken on line 2 2of Fig. 1 and shows the recesses in the curved member.

Fig. 3 is a view of a mask which is placed over the non-milledstructural member in Fig. l and protects the upper surface of saidstructural member, except for the two'rectangular shaped openingstherein, from being attacked by the milling solution.

Fig. 4 is a longitudinal vertical cross sectional view Vof this curvedraw material from which the structural member is prepared andillustrates this raw material as being covered with a protective maskingcoating except where the rectangular shaped recesses are to be formed.

Fig. 5, on an enlarged scale, shows the formation of a recess in metalwith the aid of a masking material coverving part of the metal.

Fig. 6 is an end elevation view of a curved structural member prior tochemical milling.

Fig. 7 is an end elevation view of the member in Figure 6 after beingchemically milled to form a section for use in fabricating a fuel tank,and is taken on line 7-7 of Figure 8; and,

Fig. 8 is a plan view looking down on the chemically milled section ofFigure 7.

In our chemical milling solution we provide an aqueroded by the chemicalmilling solution, i.e., all of the alloying elements of the steel areetched at the same rate as the iron or the base material of the steel.The hydrochloric acid appears to play a very active role in the chemicalmilling process. Also, the combination of the hydrochloric acid and thenitric acid appears to result in the aqua regia effect and plays anactive role in the chemical milling The addition of ferrie chloride tohycpfrate. This is espeally so in regard to Bie corrosion of the iron inthe steel. However, the ferrie chloride, hydrochloric acid and nitricacid apparently react with the iron in the steel more quickly than theyreact with the alloying elements such as silicon, nickel and molybdenum.Therefore, to increase the reaction rate of the composition with thesealloying elements there is added ammonium chlorostannate or a mixture ofstannic chloride and ammonium chloride. The addition of ammoniumchlorostannate or the ammonium chloride and stannic chloride mixturemakes it possible for the milling solution to substantially uniformlycorrode the martensitic steel. Therefore, both the base material, iron,and the alloying elements such as silicon, nickel and molybdenum areevenly eaten or corroded away. The moderator or buter, having anionization constant in the range of 10-4 ltl-e with respect to theprimary ionization constant of the hydrogen ion, appears to play therole of prolonging the active life of the milling solution. Moreparticularly, in a solution not having a moderator present the millingrate decreases almost immediately and continues to decrease at asubstantially uniform rate throughout the life of the chemical millingsolution. As is readily appreciated, with this decrease in the chemicalmilling rate the controlling of the milling or corrosion of the metalbeing reacted upon is difficult. Also, there is an economical loss inthe prolonging of the period of time the metal is in the solution as themilling bath is not being advantageously employed. However, with theincorporation of the moderator the reaction proceeds at the same uniformrate for a relatively long period of time. Again, it is seen 4that with-the reaction at a relatively uniform rate for a long period of timethat it is possible to more readily control the milling of the metalthereby realizing a more desirable product as well as being economicallyfeasible to mill the metal. In regard to the wetting agent, such as analkyl aryl sulfonate, i.e., dodecyl benzene sulfonate, this materialmakes it possible to achieve a better surface. The wetting agentprobably functions by reducing the surface tension soas to allow thegases produced by the action of the solution on the metal to morereadily escape from the surface and therefore to permit a more uniformconcentration of milling solution to react with the surface of themetal. It is realized that if the gases do not readily escape then thatportion of the metal covered by them will not be reacted upon the sameas that portion not covered by the gase bubbles and therefore therewould result an uneven or a less smooth surface.

In the milling solutions we prepared the nitrate was added as hydrogennitrate or nitric acid, the chloride as hydrogen chloride orhydrochloric acid, the acetate as hydrogen acetate or acetic acid, thecitrate as hydrogen citrate or citric acid, the borate as hydrogenborate or boric acid, and the oxalate as oxalic acid or hydrogenoxalate.

In the milling process there are a number of factors determining themilling rate. Some of these factors are temperature, concentration ofcomponents, heat transfer, evaporation, agitation, volume of millingsolution to surface area of metal, and so forth. Briefly, it is readilyrealized that the role of the temperature is very important. Generallyspeaking a ten degree rise in temperature, on the Kelvin scale,indicates a doubling in the reaction rate. Therefore, it is possible toachieve with a 'relatively dilute or weak milling solution a millingrate at a higher temperature equal t the milling rate of a moreconcentrated solution at a lower temperature. Also, it is apparent thatthe concentration of the reactant plays a very important and vital rolein the chemical mill- 6 trated ntn'c acid of a specic gravity ofapproximately 1.42 and of a concentration of 15S-70%; hydrochloric acidof a specific gravity of approximately 1.24 and a concentration of 38%;hydrofluoric acid of a specific ing. In conjunction with the temperatureeffect it is gravity 0i 1-19 @11d 50% by Weighf gia-Cial acetic midpossible by the use of a concentrated mining solution to 0f s spcltcgravity 0f 1-055.. Also, there were preparqd achieve at a lowertemperature the reaction rate equal to a func acld solution an Oxahcacfd solutlon and a bone that of a relatively dilute solution at ahigher temperaacldllltwn- These three solutions were. prepared by ture.This is of importance in the fact that a control of heating wat? to sbolhrfg Pqmt and adding an exces the milling can be obtained by controlof the tempera- 10 f c1mc amd to one bone ald to another and oxahc ture.Heat transfer is of utmost importance as good heat acid t 'the other'The Solutions were alimfed to cool transfer precludes the possibility ofa local buildup of 1 room tempflature and the superman? himd decamedtemperature and also is essential for the proper control from th?,Precllltate Example .of preplpltatlon hardened of the milling rate. Thelocal buildup of temperature lfnlrtenmc Stiuies and femm stainlesssteels are as means that in that particular area the reaction rate willowi'. prclpnauogoharilned 17"7PH and n fiPH be higher and more metalwill be removed or corroded cnlplnsm non cfu n s1 .won nfganeselchromium away. This uneven removal of the metal can result in 211%6 scpartensmstal. estsh stee s. are hopes uneven surfaces, pitting andtapering. Evaporation is of Smcon maanese an mothr llanrs ae importancein that the evaporation of a vital component, viz., such as a chloride,means that the reaction rate will tsyges 13114 44g43gss"s44octh S'S" I'CIF change in the life of the milling solution and the uniformo'f art"st ls 'hansAE 4000 er xanlg ity of the reaction rate cannot be asclosely controlled and 434% l c ee are e senes Suc as als if evaporationhad not taken place. Of great importance in the chemical milling ofmetal is the agitation of emmgnovfthhna isspetgleemafigis n: thesolution. A well agitated solution gives more unithese exam les are b waof illustration and teach; form results than one which is not agitated.This agitaand are notpmeant to e linitations on the invention I tion maybein the form of mechanical agitation whereby the main these exampleswere nm in a series of four the metal undergoing the milling action isvibrated or or ve The concentration of the components was varied shakenback and forth in the milling bath. Another form to detrmine the inuenceof the different components on of agitation may be that of the millingsolution itself the reaction rate In order to arrive at reaction ratesfor whereby it is agitated by gas bubpling through it' .Anq averagetemperature it was decided that the base tema still further and desirable form is that of ultrasonic vipermute should be in the range of150 1 55 F. In Some Elrtllii ghb'olillltgill'lffgutn'eunldgnrs instancesthe temperlature of the reaction solution was a roximate 140 In thaction so as to break up any localized variations in the ge of 160170 p.Andinegrlstnsola; 3;: solution'concentraion. tllliis alrsfoservetoilivoid gas about 200 F. I t the temperature were approximatelycllmu M1011 0n me a SU ace; u emolfe e 140 F. the reaction rate wasincreased by about 25% um o e vol e f the r acting chemical millingsolution in pro to convert n to the rango of about 150.155 E Llkoportionto thebsurfce aref Ofblil material Undelf'oillg 40 wise, if thetemperature were in the range of 160-175" F. reaction can c O COUSI eraimpoftan t e the milling rate was decreased about 25% to brin it volumeto surface ratio is rather 10W it iS PQSSibie that in the 15G-155 F.range. It is to be appreciated hat the reactive components available atthe milling surface this is an approximation In tho .tblos the millingfoto will be rapidly exhausted and therefore there will be an for theiodio-ated temperature is given and the milling uneven reaction rate andpossibly an uneven removal of rato for the base .temperature rango isgivoo in parenthe metal aieclng the quality 0f the finished Product 45theses. The metals milled bythe various solutions were Therefore, alarge ratio of volume to surface area will AM350 and 17.7PH, Thesmoothness of .tho mined give a more constant reaction rate and thuspermit closer moral is expressed in root mean square terms, R.M.S.,tolerances on the finished product. and the unit of measurement is microinches of amplitude To more particularlyillustrate our discovery andespeof surface variation. cially the effects of the variation of the-concentration of In Table I there are presented four dilerent solutionscomponents and also the effect of the variation of the temand theresults of milling with these solutions. These perature we herewithpresent specific detailed examples. solutions basically comprised water,hydrochloric acid These examples are to be taken for illustrativepurposes and ferrie chloride. In solution D there was also nitric onlyand are not to be taken as limitations on the in- 5r acid. In this tablethe hydrochloric and nitric acids vention. In these examples thespecifically enumerated are expressed in terms of normality and theferrie chloride components were utilized. More particularly, concenisexpressed in molarity. These solutions were used to Table I A B C DVariables Wt. Per- N Wt. Per- N Wt. Per- N Wt. Percent N cent cent centHoi as 12.1 1.45 19.8 2.58 18.5 2.58 o ((d) 9.8 0.09 10.0 3.7 22.3 i asn.01 18.2 .2 "air :1:::2 631% P- T? 100.1 100.0 100.3 100.1 ieasss'rri43350'-" ii350-" trimm dso '"H" Mining Rat'e (ixii's 0.06664'I1 AM350-00602.-... 0.066`(1&i01'3`50')';"` mi' (150 F (iggoii (150 F uiiiiioaosi(isb-Jim (iggoig, F., R.M.s s0-100 10o-120---. iis-'50...)

mill AM350 at approximately 140 F. Comparison of solutions A, B and Cshows that C milled faster than B which in turn milled faster than A.More particularly, C milled at about 0.0002 inch per minute as comparedwith B at 0.0001 and A at 0.00004 inch per minute. The concentration ofthe ferric chloride was increased from A through C. And, theconcentration of the hydrochloric acid in both C and B was approximately50% greater than in A. Turning; now .to solution D it is seen that theaddition of the nitric acid considerably increased the reaction rate.Solution D may be considered to be very similar to solution B with thereplacement of some of the water with nitric acid. The milling rate of Dwas about 0.0006 inch per minute as compared with approximately 0.0001inch per minute for B. This increase in the reaction rate may beconsidered to be partially due to the increased total acid concentrationin D. In summary, it may be stated that an increase in theconcensolutions .the concentrations of all the components were in thesame range, varying from about 2.35 to about 3.36 normal for thehydrochloric acid, Afrom about 0.69 to about 0.75 normal `for the nitricacid, and from about 1.61 to approximately 2.3 molar for the ferricchloride. lFrom this it is possible to state that a moderate variationin the concentration of the components does not appreciably affect themilling rates of the solutions. However, turning to solution D it isseen that the concentration of the nitric acid was increased appreciablyto about 1.44 normal. The milling rate for this solution increased toabout 0.0005 inch per minute or about 60%. Again, it is seen that anincrease in the concentration of the nitric acid within reasonablelimits, appreciably increases the milling rate. An alkyl aryl sulfonatewas employed to assist in the reaction. More particularly, to permitgreater penetration of solution to .the surface of the metal.

Table II A B O D b vm l wa. P6:- N Wsrer- N wsrer- N WsPer- N cent centcent cent Meta] AM350-... AM350.-. AM350 AM350..-. Temperature, F 140140 140 un Ml11lngRate(inches/m1n.) 0'0003 o R.M.S 70 70 tration of vtheferrie ion increases the milling rate. Also, an increase in the 4totalacid concentration increases the milling rate. As a sidelight, solutio'nD was used to mill 17-7PH steel as well as AM350. The milling rate of17-7PH steel was approximately 0.00035 inch per minute as compared with0.0006 inch per minute for AM350.

Turning now to Table II another series of solutions was preparedcomprising water, ferrie chloride, hydrochloric acid and nitric acid.The concentrations of the components in 4these solutions were varied tobring forth the effects of the components on the milling rate. In thistable the concentration of hydrochloric and nitric acids is indicated bynormality while the concentration of the ferrie chloride is expressed bymolar-ity. A comparison of the milling rates of solutions A, B and C onAM350 at about 140 F. shows that they were substantially the Turning nowto Table III another series of milling solutions was prepared. In thisseries the concentration of the nitric acid was varied over a Widerange. Again the components were water, ferrie chloride, hydrochloricacid and nitric acid. The hydrochloric and nitric acids were expressedin terms of normality and the ferrie chloride in terms of molarity. Themetal milled was AM350 and at a temperature of about 140 F. In thisseries of solutions solution A was solution D of Table I. Theconcentrations of the hydrochloric acid and the ferrie chloride weremaintained in about the same range, i.e., the hydrochloric acid variedfrom about 1.98 normal to about 2.35 normal and the ferric chloridevaried from about` 4.42 to 1.68 molar. However, the concentration of thenitric acid was increased from 1.44 normal in solution A to 3.64 normalin solution E. The milling rate of solusame, i.e., about 0.0003 inch perminute. In these three tion A was the lowest, 0.0005 inch per minute. Insolu- Table III .4 B o D E Variables Wt. Per- N Wt. Per- N Wt. Per- NWt. Per- N Wt. Per- N cent 0011i; 0B Cent Dent nc1(as%) 16.7 2. 35 15.92.24 15.2 2.16 14.6 2.07 14.0 1.98 tgcl). (g1g .76% 1.22 1.64 17.6 1.4816.9 1.42 11a/fm1 maar.; 1: :1 :1 M3 mi if?. 24:? L 11,0 53.6 51.1 48.946.9 44.9 l-f 100.1 99.9 100.0 100.0 99.9

Metal AM350. A141360... AM350 AM 5 Temperature GF.) 140 140 140 140 3 Oiiiom Mining Bate (inches/mm.) 0. 0005.--.. 0.00067 0.00082 000053-...0.00066 an... 90s-.1. 2 150 RMR m. M. gooo) 6{151.00070} ,action rate.

tion B the milling rate was 0.00057 inch per minute, a slight increaseover solution A. In solution C, with a concentration of nitric acid at2.63 normal the milling rate was the highest for the series, 0.00082inch per minute. With a further increase in the concentration of thenitric acid, see solutions D and E, the milling rate decreased toapproximately that for solutions A and B. This indicates that theincrease in the concentration of nitric acid is beneficial to a certainconcentration and about 1Z0-140 F. Solution A was the fundamental solution. Solution B was similar to solution A except that theconcentration of the ferrie chloride was increased approximately by onethird. Solution C was similar to solution A except that there was addedferric nitrate so as to increase the concentration of the fern'c salt.And, solution D was also similar to A except that there was added ferricsulfate to increase Athe concentration of the ferrie salt. The millingrate of all of these solutions was then it is of no further value andactually retards the re- 10 in the same range, i.e., 0.0003 to 0.0006inch per minute.

TableIV .1. B c D Variables Wt. Per- N Wt. Percent N Wt. Per- N Wt. Per-N 0811i; Cent cent Home?) 160 2.34 155 2.34 15.6 2.34 15.6 2.34 F0011(202 1.65 252 2.24 18.9 1.65 18.9 1.65 HNO| 6870 l,) 95 1.43 90 1.43 8.91.43 8.9 1.43 0:1(M 6.3 1.24 Feo (M 6.3 0.75y H 53.0 50.3 50.3 50.3

Metal.. M350 M350 I A11/1350.... AM350 Temperature F.) 12o-140.--.{fffll 12o-140..- x20-140.-. Mnnngmte(mehes/m1n.).. 00005.... O'ggg-:0.0006,... 00003-..-

(160F (150F.. (12o-140F, (150216., (150 F., 0. 00075.) 0. 090g; 0.0009.) 0. 00045.)

0000525 R.M.s---

More particularly, it may be surmised that too high a concentration ofthe nitric acid passivates the surface of the metal being milled.Possibly there is formed an oxide layer on the surface which acts as amembrane to decrease the ow of active components in the solution to themetal. Again, an alkyl aryl sulfonate such as the sodium salt of dodecylbenzene sulfonic acid was incorporated into the aqueous reactionsolution. This salt appears to have decreased the surface tension of thesolution so as to allow the same to penetrate to the surface of themetal for greater ease of reaction.

There was prepared another series of milling solutions, see Table IV, inwhich -the concentration of the ferrie salt was varied and in whichdiierent ferrie salts were employed. The fundamental solution comprisedwater,

In Table V there is illustrated another series of milling solutions.Again, the fundamental solution comprised water, ferrie chloride,hydrochloric and nitric acids. The concentrations of the hydrochloricand nitric acids are expressed in terms of normality and theconcentration of the ferric chloride is in terms of molarity. Thesesolutions were employed to mill AM350 in the temperature range of aboutl60-170 F. Solution A was the funda mental solution. Solution B wassimilar to solution A but comprised about 3.2% by weight of acetic acid.Solution C was similar to A except that it comprised about 3.2% byweight of glycerine. The milling rates of A and B were substantially thesame, 0.0008 and 0.0007 inch per minute. The milling rate of solution Cwas considerably less than these two as it was 0.0002 inch ferriechloride, hydrochloric acid and nitric acid. This per minute.

Table V .A B C Variables Wt. per- N Wt. per N Wt. per- N cent cent centMetal AM350-.- AM350.. AM350... Temperature, F 1GO-170 i60-170...160-170 Milling Rate (inches/mln.) 0.0008 0. 0007...-. 0.0002

(150 F., (150 F., (150 F.,

0. 0006.) 0. 00053) 0. 00015) R M n 65 100 Another series of millingsolutions was prepared com prising water, ferrie chloride, hydrochloricand nitric acids. The results of this series are presented in Table VI.To this fundamental solution there was added ammonium chloride, stannicchloride and ammonium chloroemployed to mill AM350 at a temperatureinthe range of stannate. In this table the ferrie chloride, ammonium 11chloride, stannic chloride and ammonium chlorostannate were expressed interms of molarity and the nitric acid and hydrochloric acid in terms ofnormality. These solutions were employed to mill AM350 in thetemperature range of about 1GO-170 F. The fundamental or controlsolution was solution A. 'Ihe milling rate of this solution was about0.0005 inch per minute. Solution B was similar to A except that therewas added ammonium chloride. The milling rate of solution B was 0.0008inch per minute, an increase of about 35% over the 412 of E was slightlylower than D, due to the lower milling temperature. Solution E milledAM350 at 0.0011 inch per minute, faster than it milled 17-7PH, 0.00025inch per minute. In summary, the addition of ammonium chloride to thefundamental solution increased the reaction rate. .The addition ofstannic chloride increased the reaction rate almost three fold over thatof the fundamental solution. And, the addition of both ammonium chlorideand stannic chloride increased the reaction over that of the fundamentalsolution and also gave a smoother millmg rate of A. Solution C was alsosimilar to A surface than the control solution gave.

Table VI A B C D E variables Wt. Per- N Wt. Per- N Wt. Per- N Wt. Per- NWt. Per N cent cent cent cent cent Hmmm 15.0 2.34 147 2.34 14.7 2.34 147234 14.7 2.34 Fem. (M) 25.2 2.23 23.0 2.23 230 2.a 23.5 2z; 23.0 2.23HNo.s-70%) 8.9 1.44 a4 1.44 8.4 1.44 8.4 144 9.4 1.44 13,0 50.3 47.347.3 47.3 47. 3 NBtol (M) 5.9 1.7 snc), (M) 5.9 0.4

(NH 0'4 (Nmiil) 0'4 SChzNHClm- 4.4 0.25 4.4 "6.'25 (SnCh) (SnCl MetalAM350.. M350 M350 AM350 411143725,H Temperature F...) 10o-170 16o-1705%14 agp-1Z0 152%?) Milling Ratetlnches/min.) 0.0005 0. 0008 0:000:0602) 0.00112 (150 F., (150 F., (150 F., (150 F., 17-7PH,

0.00033.) 0.00068.) 'R M Ft 60 45 m except that there was incorporatedstannic chloride. The milling rate for C, 0.0014 inch per minute, wasalmost three times the milling rate of A, 0.0005 inch per minute. As isseen, the addition of the stannic chloride considerably increased themilling rate. Solution D combined all the components so that it wassimilar to A but with the addition of ammonium chloride and stannicchloride. The ammonium chloride and stannic chloride can be consideredto be ammonium chlorostannate. The milling rate of D was comparable tothat of C, 0.0013 inch per minute. In 4the table the ammonium chlorideand the stannic chloride in solution D were expressed as being separatebut it is to be realized that in actuality they were in the form ofammonium chlorostannate. Solution E was substantially the same assolution D. However, the milling temperature of E was approximatelylower than for D. Also, solution E was used to mill both AM350 and17-7PH steels. The milling rate In Table VII there is presented anotherset of milling solutions having Water, ferrie chloride, nitric acid and"hydrochloric acid. This fundamental solution was varied by the additionof boric acid, phosphoric acid or oxalic acid. In this table thehydrochloric and nitric acids are expressed in terms of normality andthe ferrie chloride, boric acid, phosphoric and oxalic acids areexpressed in terms of molarity. 'Ihese solutions were used to mill AM350in the temperature range of about 160-170 F. Solution A was the controlsolution and milled at the rate of about 0.0007 inch per minute.Solution B was similar to A except that there was added boric acid andit milled at the rate of about 0.0008 inch per miuute. VSolution C wassimilar to A except that it contained phosphoric acid and milled at therate of approximately 0.0007 inch per minute. Solution D was similar toA but contained oxalic acid and milled at the rate of about 0.0006 inchper minute. Also,'solution D was employed Table VII AA B C D VariablesWt. Per N Wt. Per- N Wt. Per- N Wt. Per- N cent cent cent cent HCl (38??15. 6 2. 34 14. 7 2. 34 14. 7 2. 34 14. 7 2. 34 FeClg (Lf 25. 2 2. 2323.6 2.23 23.6 2.23 23.6 2.23 HNO; (S8-70% 8.9 1.44 8.4 1.44 8.4 1. 448.4 1.44 H,O 50. 3 47. 3 47. 3 47. 3 Boric Acid (M) 5. 9 1. 47Phosphoric Acid (M)' 5. 9 9 Oxalic Acid (M) 5. 9 1.0

Metal AM350 AM350.... M350 Allviiggh Temperature, F 160-170 160-170i60-170-... i60-170 Milling Rate (inches/min.). 0.0007 0.0008 0.0007

( r., (150 F., (150 F., (150 F.,

17-7PH 0.0005 3353. n M s 60 50 50 M 13 to mill 17-7PH steel with amilling rate of about 0.0005 inch per minute.

Table VIII presents the summary of the results with two millingsolutions. These solutions comprised water, nitric acid, citric acid,hydrochloric acid, hydrolluoric acid, acetic acid and disodiummonohydrogen phosphate. Solution B4 dilered from A in that B contained aferrie salt, ferric nitrate. In this table the ferrie salt is expressedin terms of molarity and the concentration of the nitric acid, aceticacid, hydrochloric and hydrolluoric acids are expressed in terms ofnormality. These solutions were employed to mill 177PH steel atapproximately room temperature. Solution A, without the beneiit of theferrie salt, milled at the rate of 0.0001 inch per minute. Solution B,with the benefit of the ferrie salt, milled at the rate of 0.0003 inchper minute. From these results it is possible to state that the additionof a ferric salt increases the milling rate of the solution on 17-7PHmetal.

Table VIII I A B Variables Wt. Per- N Wt. Per N cent cent HNO. (B8-70%)20.9 2.58 20. Aoetic Acid (glacial) 2. 3 2. HCl (38%) 9.2 9. BF (60%13.1 13. Citric Acid (sat 0.6 0. NaiHPO (satd). 0.6 0. H10 53. 5 62.Fe(Nx)| (Ml 1. 8 0.07

Metal 17-7PH 17-7PH Temperature, F Room Room Milling Rate (inches/min.)0. 000 0.0003 R.M.S 80-100 i90-250 Taxle IX presents the results ofmilling with two other milling solutions. These solutions comprisedwater, nitric acid, acetic acid, hydrochloric acid, hydrouoric acid,hydrobromic acid, citric acid and disodium monohydrogen phosphate. Inthis table the concentrations of the ferrie salt is expressed in termsof molarity and the concentrations of the nitric acid, acetic acid,hydrochloric acid, hydrouoric acid and hydrobromic acid are in terms ofnormality. These solutions were used to mill l7-7PH steel atapproximately room temperature. Solution B van'ed from solution A inthat the former contained a ferric salt, ferrie nitrate. Solution A,without the benefit of ferrie salt, milled this steel at the rate of0.0001 inch per minute. Solution B, with the benefit of the ferrie salt,milled this metal at the rate of 0.00022 .inch per minute. Again, it ispossible to state that the addition of a ferrie salt to a millingsolution increases the milling rate on martensitic steel.

Table IX v A B Variables Wt. Per- N Wt. Per- N cent cent HNO: (G8-707)19. 8 2. 48 19. 5 2. 48 acetic Acid (glacial) 2. 2 o. 4 2.1 o. 4 l (387)8.8 1. 02 8. 8 1. 02 12. 4 4.2 12.2 4.2 5. 1 0. 3 5. 0 0. 3 0. 0.5 0. 50. 5 H2O 50. 6 49. 8 Fe(N0t)| (M)- l. 1 0.7

Metal 177PH 17-7PH Temperature, F Room Milling Rate (inches/m 0.

................ ISO-230 Table X presents the summary of milling withtwo other milling solutions. These solutions comprised water, nitricacid, acetic acid, hydrouoric acid, citric acid, and disodiummonohydrogen phosphate. In this table the concentration of the ferricsalt is expressed in terms of molarity and the concentrations of thenitric acid, acetic acid and hydroiluoric acid are in terms ofnormality. These soltuions were employed to mill 17-7PH metal atapproximately room temperature. Solution B differed from A in that theformer comprised ferrie nitrate while the latter did not. The millingrate of solution A, without benefit ofthe ferric salt, was 0.00002 inchper minute. The milling rate of solution B, with benefit of the ferriesalt, was 0.00008 inch per minute. Again it is possible to state thatthe addition of a ferrie salt to a milling solution increases themilling rate of the solution on martensitic steel.

Table X Variables HNO; (0S-70%) Ateticoacid (glacial).

Metal 17-7PH At this time it is proper to compare the results of themilling solutions in Tables VIII, IX and X. The milling solutions inTables VIII and IX comprised hydrochloric acid. The milling solutions inTable X did not have the benefit of the hydrochloric acid. It is seenthat the milling solution comprising hydrochloric acid milled at anincreased rate over the milling solutions not having hydrochloric acid.More particularly, in Tables VIII and IX the milling solutions havinghydrochloric acid but not having a ferric salt milled at the rate of0.0001 inch per minute. A corresponding solution in Table X not havinghydrochloric acid and not having the benefit of a ferrie salt milled atthe rate of 0.00002 inch per minute. In other words, the millingsolution without benefit of hydrochloric acid milled at a rate ofone-sixth of those solutions having the benet of hydrochloric acid.Furthermore, for those milling solutions having both hydrochloric acidand the benefit of ferrie ion the milling rate was greater than for themilling solutions not having either hydrochloric acid or ferric ion.Solution B in Table IX, having both hydrochloric acid and ferrie ion,had a milling rate of 0.00022 inch per minute. Solution B in Table VIII,having both hydrochloric acid and ferrie ion, had a milling rate ofabout 0.0003 inch per minute. Solution B in Table X, having neitherhydrochloric acid nor ferrie salt, had a milling rate of about 0.00008inch per minute. It is seen that the milling rates of solutions B inTables VIII and IX are at least ten times the milling rate of solution Bin Table X. These two series of data illustrate the value ofhydrochloric acid in a milling solution.

A further series of milling solution was prepared. This series comprisedwater, nitric acid, acetic acid, hydrouoric acid, citric acid, disodiummonohydrogen phosphate and fernc nitrate. In this table the nitric acid,hydrouoric acid and acetic acid are expressed in terms of normality andthe ferrie salt in terms of molarity. These solutions were used to milll7-7PH steel at various temperatures ranging from room temperature toabout 200 F. The fundamental solution was A and 15 comprised water,nitric acid, acetic acid, hydrouoric acid, citric acid, disodiummonohydrogen phosphate and ferric chloride. This solution milled at therate of 0.00014 inch per minute. To a solution such as A there was to bechemically milled and is dipped or submerged in the milling solution fora predetermined time. With the passing of the time the milling solutioneats away or corrodes away the metal so as to form the two recesses 11.

attached other protective neoprene iilms. A crosssectional view of thestructural panel before chemical milling takes place is shown in Figure4 whereby the panel is covered by the neoprene having the two cutouts 14added ferrie nitrate to form B. Solution B milled at 5 It is to be notedthat that portion of the metal covered by the rate of 0.00033 inch perminute. The milling rate the protective neoprene tilm is not attackedand only that of B was over twice the rate of A. From this itispossiportion of the metal not covered by the protective lm is ble tostate that the addition of ferrie nitrate to a millcorroded away. Afterthe passage of a predetermined ing solution, or the increase oftheferrie ion in. the millof time and the recesses are of a sut'licientdepth and ing solution, increases the milling rate. To a solution sizethe structural panel is removed from the milling such as B there wasaddedn powdered titanium to form solution. The adherent solution iswashed away and C. Solution C milled 177PH steel at temperatures of theneoprene protective lm stripped otf the panel to pro- 80 and 145 F. Asthe milling temperatures of A and duce that shown in Figure l. Otherteachings for the B were room temperatures, it is not possible tocompare masking of metal may be found in United States Patents themilling rate of C with these. However, it is seen Numbers 2,739,047 toSanz and 2,684,291 to Wilsonet al. that at 80 F. the milling rate waslower than for A In Figure 5 there is illustrated the action of themilling and B and at 145 F. the milling rate was greater than solutionon the metal at the junction of the protective for A and B. To asolution such as C there was added lm and the metal. IIt is seen thatmilling solution 16 eats aluminum nitrate to form solution D. Thissolution away metal 17 so as to undercut protective film 18. One milledat a temperature of 135 F. and the milling rate 20 of the particularadvantages of the use of a wetting agent was 0.0011 inch per minute. Itis seen that the addiis to induce better contact between the millingsolution tion of aluminum nitrate to the milling solution increased andthe metal. With a better contact or a lowering of the milling rate. To asolution such as D there was addthe effective surface tension there isless possibility of ed a powdered aluminum alloy, 75S, to make solutionE. gases collecting on the surface or near masked edges un- The millingtemperature of this solution was 200 F. and 25 derneath the protectivelm. For this function there are the rate was 0.00026 inch per minute.These results are a number of suitable wetting agents such as pine oil.tabulated inTable XI. One of the common wetting agents is an alkyl arylsul- Table XI A B O D E Variables Wt.Per N Wt. Per- N Wt.Pe1-nt N Wt.PerN wmer- N cent cent cent cent HNO. (6s-70%)---" 20.2 2.48 19.2 2. 4s19.7 2.48 19.5 2.48 19.3 ats Aeetic acid (glacial)- 2.2 0.4 2.2 0.4 2.10.4 2.1 0.4 2.1 0.4 HF (00 o 12.6 4.2 12.4 4.2 12.3 4.2 12.2 4.2 12.14.2 citric Acid (sand) 0.6 0.6 0.6 0.5 0.5 NaiHPo. (sand). 0.0 0.6 0.00.5 0.5 11,0 52.1 61.0 00.6 00.0 59.4 Feci. (M) 1.2 0.12 1.a 0.12 1.70.12 1.7 0.12 1.7 FetNom (M) 1.8 0.07 1.7 0.07 1.1 0.07 1.7 0.7 0.7 0.7AnNon. 1.0 1.0 75 s 0. 8 Metal. 17-7PH 17-7PH 17-7PH 17-7PH 17-7PHTemperature F.) Room Room 80, 5 235 200 Mining nate (inches/ming--. 0.000144 0. 00033 ljjmg 0.0011 0.00025 R.M.s 00-110 13o-180 j 10o-15090-130 Having described our discovery and illustrated the same fonate.Specific sulfonates which are useful are the with reference to variouscompositions of matter for the sodium salts of dodecyl benzene sulfonicacid and use of chemical milling solution, reference is hereby madepentadecyl benzene sulfonic acid. to the practical application of thesesolutions. Referring Another instance where our solution is of value isin to the drawings, and especially Figure l, it is seen that thechemical milling of components for aircraft. |In Figthere is illustrateda structural panel 10 having two chemi- 55 ure 6 is illustrated a curvedstructural plate having a thickcally milled recesses 11 therein. InFigure 2, a vertical ness of 0.750 inch. This plate was transformed intoa cross-sectional view of Figure l. it is seen that this struestructuralmember, for use in a fuel tank, having a thicktural panel is curved andthat these two recesses are ness of 0.100 inch. The finished Product,See Figures separated from each other by a rib 12. In the manufac- 7 and3, W28 Prepared from the Original Structural Plate ture of this type ofstructural panel the material to be 60 by masking the two centralportions so as to form the boss saved is covered by an adhering andstrippable protective upon the corrosion of the surrounding metal. As isilm or coating, there are numerous such coatings or tilms seen, one ofthese bosses is 0.500 inch in thickness and examples being neoprene,vinyl etch-proof film, and the other boss is 0.750 in thickness. vilachof these is others. More particularly, for shallow milling there maytapped. The outside perimeter of this section is formed be used variousacid resistant paints. To pictorially llusinto a thickness of 0.250inch, approximately two and trate the making of this structural panelreference is made one-half times as thick as the main part of thesection.

. to Figures 3 and 4. In Figure 3 there is illustrated a neolll'heincreased thickness of the perimeter is for added prene mask 13 havingtwo recesses 14 therein. This neostrength upon the welding of thissection into the main prene film 13 is placed on the concave side of thestrucpart ofthe fuel tank. Normally such a section would be tural panel10 and on the edges and the convex side are 70 formed by fabricationusing a welding technique. However, the welding process causes graingrowth which in turn causes a weakening of the metal. With our methodthere is no grain growth.

The control for determining the degree of chemical illustrated therein.This structural panel now ready '(0 may be carried out in a number ofditferent man- 17 ners. One of these is to introduce a material of apredetermined thickness into the chemical milling solution along withthe member to be chemically milled and to periodically weigh or measurethis member. When this member has decreased a certain amount in weightor thickness it is possible to decide Whether the structural memberundergoing chemical milling has been corroded away suiciently.

Returning now to the metals and the examples presented therein it isnoticed that the smoothness of R.M.S. values are mainly in the range ofapproximately 50-125 R.M.S. This is of a special importance as thisvalue represents a surface that is relatively smooth. In fact, themilled metal is often smoother than the metal when machine milled. Inmachine milling the average smoothness value is approximately 125 R.M.S.Of course, this value of 125 R.M.S. is subject to variation as with softalloys it is possible to machine mill the metal somewhat smoother than125 R.M.S., but with heat treated, work hardened and tempered alloyswhich may have a tough surface skin it is difficult to achieve asmoothness value of 125 R.M.S. on machine milling. However, with thischemical milling process it is possible to achieve a smoothness valuewhich approaches that used for mirrors and reflective surfaces, and itis possible to achieve this smoothness on all types of alloys be theyhard, heat treated, work hardened, or tempered.

The smoothness values were determined by a protilometer. Theprofilometer used possessed a diamond needle which rode on the surfaceof the metal and measured the profile of the surface in millionths of aninch in amplitude. These measurements are expressed in R.M.S. or rootmeans square values. Such an instrument is commercially obtainable fromPhysicists Research Company, Ann Arbor, Michigan,

In this chemical milling process itis noticed that there is little ifany smut. Smut is defined as that material which clings to the metalitself. Smut is usually an oxide of the alloy or chemical compounds ofthe alloy or unreacted finely divided alloying materials and isattracted to the -bulk metal by electrochemical and electrostaticforces. l

With our process in chemical milling it is possible to achievetolerances in some usages which have not been obtainable in commercialpractice before. These tolerances are in the range of approximately plusor minus 0.0005 inch, making it possible to achieve a high quality and/ra precision product. Also, in our process it has been observed thatthere is no intergranular corrosion due to the chemical action of thesolutions. This success in not having intergranular corrosion makes itpossible to achieve higher strength components for less weight of thecomponent as compared with the presently machine milled and weldedcomponents.

Our process is also adaptable for a continuous method or a batch method.In the continuous process it is possible tocontact the structural itemto be made with the milling solution and to continuously recirculate themilling solution so as to maintain uniformity of solution in contactwith all reacting surfaces. Chemical processing equipment, i.e., tanks,pipe fittings, valves, and pumps lined with the suitable resistantmaterial such as polyethylene, polyvinyl chloride, and polyvinyl esters,capable of withstanding the chemical action of these solutions areavailable. Also filter screens and heat exchange equipment to withstandthe chemical action of these solutions are available. Therefore it ispossible to maintain close Ysolution control necessary for continuousoperation.

Examples of other ways and means for chemically milling metals areherewith presented. A metal may be removed by spraying the solution ontoit so as to have a fine stream of the reactant solution contact themetal. By this technique deep grooving can be realized by rotating apart. In another manner a pipe may be made lighterin-weight by runningthe solution through it so as to de- 18 crease the wall thickness. As isrealized the interior wall of the pipe is eaten away and the insidediameter thereby enlarged.

From the above description of our discovery it is seen that the same isapplicable to the chemical milling of metals and alloys. From ourdiscovery it is possible to save considerable waste in the manufactureof metal products; to produce the parts in batches or continuously; thecapital investment for production of the components may be as low asapproximately 5 to 10 percent of the capital investment for equivalentmachine milling. Also, it is possible to produce highly complex shapesand congurations with chemical milling which are not possible withmachine milling, to use non-symmetrical patterns with chemical millingwhich are dicult to use With machine4 milling, to produce integrallystilened structures whereby one unit serves as a structure instead of anintegrally fabricated structure depending upon welds, which are weakerthan the metal itself, to produce a structure, and to make taperedstructural materials which are diflicult to make with machine milling.Furthermore, from an engineering design standpoint it is seen that it ispossible to etch after forming the structure. In many operations it iseconomically feasible to obtain closer tolerances, to have a number ofvarious steps to cut, and to mill all surfaces of the partsimultaneously. In this regard, it is possible to chemically mill withequal ease all types of martensitic steel. From a labor standpoint andskilled artisan viewpoint it is not necessary in chemical milling to useas highly skilled operators as in machine milling.

Having described our discovery, what we claim and wish to protect is asfollows:

1. An aqueous acidic composition of matter adapted for chemicallymilling metal which contains chloride, nitrate and a ferric ion therein,the chloride concentration being at least about 1.0 normal, the nitrateconcentration being at least about 0.75 normal, the ferrie concentrationbeing at least about .07 molar, the available hydrogen ion concentrationbeing however, at least 1.8 normal, said composition further including aminor amount of a member selected from the group consisting of ammoniumchloride and stannic chloride, ammonium chlorostannate and mixturethereof.

2. The composition of claim 1 containing also a weak acid having aprimary ionization constant in the range of 10-4 to 10-5.

3. An aqueous acidic composition of matter adapted for chemicallymilling metal and which contains a chloride, a nitrate and a ferrie iontherein, the chloride concentration computed as 38% HCl being at leastabout 9% by weight, the nitrate concentration computed as 70% nitricacid being at least about 5%, the ferrie concentration computed asferric chloride being at least about 1%, all percentages being byweight, the available hydrogen ion concentration being, however, atleast 1.8 normal, said composition further including a minor amount of amember selected from the group consisting of ammonium chloride andstannic chloride, ammonium chlorostannate and mixtures thereof.

4. An aqueous acidic composition of matter adapted for chemicallymilling metal which contains a chloride, a nitrate and a ferric iontherein, the chloride concentration computed as 38% HCl being in therange of about 9-25%, the nitrate concentration computed as 70% nitricacid being in the range of about 5-25%, the ferrie concentrationcomputed as ferrie chloride being about 1-26%, all percentages being byweight, said composition further including a minor amout of a memberselected from the group consisting of ammonium chloride and stannicchloride, ammonium chlorostannate and mixtures thereof.

5. 'Ihe composition of claim 4 containing also from about 16% of a weakacid having a primary ionization constant in the range of 10* to 1.0".

6. A process for manufacturing a metal structure from steel, saidprocess including covering portions of the steel with a protectivecovering, subjecting the uncovered steel to the action of an aqueousacidic composition of matter adapted for chemically milling metal whichcontains a chloride, aA nitrate and a ferric ion therein, the chlorideconcentration being at least about 1.0 normal, the nitrate concentrationbeing at least about 0.75 normal, the ferric concentration being atleast about .07 molar, the available hydrogen ion concentration being,however, at least 1.8 normal, said composition 'further including aminor amount of a member selected from the group consisting of ammoniumchloride and stannic chloride, ammonium chlorostannate, and mixturesthereof.

7. A process for manufacturing a metal structure from steel, saidprocess including covering portions of the steel with a protectivecovering, subjecting the uncovered steel to the -action of an aqueousacidic composition of matter adapted for chemically milling metal, whichcontains a chloride, a nitrate and ferric ion therein, the chlorideconcentration computed as 38% HCl being at least about 9% by weight, thenitrate concentration computed as 70% nitric acid being at least about5%, the ferric concentration computed as ferric chloride being at leastabout 1%, the available hydrogen ion concentration being, however, atleast 1.8 normal, said composition further including a minor amount of amember selected from the group consisting of ammonium chloride andstannic chloride, ammonium chlorostannate and mixtures thereof.

8. A process for manufacturing a metal structure from martensitic steel,said process including covering portions of the raw material metal witha protective covering and subjecting the uncovered raw material metal tothe action of an aqueous acidic composition of matter adapted forchemically milling metal, which contains chloride, nitrate and ferricmoities therein, the chloride concentration computed as 38% HC1 being inthe range of about 925%, the nitrate concentration computed as 70%nitric acid being in the range of about 5-25%, the ferric concentrationcomputed as ferric chloride being about 11-25 said composition furtherincluding a minor amount of a member selected from the group consistingof ammonium chloride and stannic chloride, ammonium chlorostannate andmixtures thereof, and from about 16% of a weak acid having a primaryionization constant n the range of 10-4 to 10-6, all percentages beingby Weight.

References Cited in the file of this patent UNITED STATES PATENTS469,704 Knidermann Mar. l, 1892 2,429,107 Petren Oct. 14, 1947 2,446,060Pray July 27, 1948 OTHER REFERENCES Thum: Book of Stainless Steel, 2nded., Amer. Soc. of Metals, Cleveland, Ohio, Jan. 1935, p. 151.

Monypenny: V. 2., 3rd ed., rev. 1954, Chapman & Hall, London, p. 253.

1. AN AQUEOUS ACIDIC COMPOSITION OF MATTER ADAPTED FOR CHEMICALLYMILLING METAL WHICH CONTAINS CHLORIDE, NITRATE AND A FERRIC ION THEREIN,THE CHLORIDE CONCENTRATION BEING AT LEAST ABOUT 1.0 NORMAL, THE NITRATECONCENTRATION BEING AT LEAST ABOUT 0.75 NORMAL, THE FERRIC CONCENTRATIONBEING AT LEAST ABOUT .07 MOLAR, THE AVAILABLE HYDROGEN ION CONCENTRATIONBEING HOWEVER, AT LEAST 1.8 NORMAL, SAID COMPOSITION FURTHER INCLUDING AMINOR AMOUNT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF AMMONIUMCHLORIDE AND STANNIC CHLORIDE, AMMONIUM CHLOROSTANNATE AND MIXTURETHEREOF.